Scratch-resistant sol-gel coating for clear powder-slurry lacquer

ABSTRACT

The present invention relates to a process for producing coated substrates, especially coated automobile bodies, coils and furniture, in which first of all a powder slurry clearcoat material and then a sol-gel clearcoat material are applied to an optionally precoated substrate and are then cured conjointly.

This application claims priority under 35 USC § 120 upon InternationalPCT Application PCT/EP 99/06607 filed Sep. 8, 1999, now WO 00/16912 andGerman Patent Application DE 198 43 581.9, filed Sep. 23, 1998.

The present invention relates to a process for producing coatedsubstrates, especially coated automobile bodies, in which first of all apowder slurry clearcoat material and then a sol-gel clearcoat materialare applied to an optionally precoated substrate and are then curedconjointly.

Automobile bodies are for the most part provided with a multicoatcoating system. As the final coat, it is common to apply clearcoatmaterials. For this purpose it has recently been possible to use whatare known as powder slurry clearcoat materials as well.

Powder slurry coating materials comprise powder coating materials in theform of aqueous dispersions. Slurries of this kind are described, forexample, in the U.S. patent U.S. Pat. No. 4,268,542 and in the Germanapplications DE-A-195 18 392.4 and 198 14 471.7, which were unpublishedat the priority date of the present specification.

Recently, materials known as sol-gel clearcoats and based onsiloxane-containing coating formulations have been developed which areobtained by hydrolysis and condensation of silane compounds. Thesecoating materials, which are used as coating compositions on plastics,are described, for example, in the German patents DE-A-43 03 570, 34 07087, 40 11 045, 40 25 215, 38 28 098, 40 20 316, and 41 22 743.

Sol-gel clearcoats impart very good scratch resistance to substratesmade of plastic, such as spectacle lenses or motorcycle helmet visors,for example. This scratch resistance is not achieved by the known OEM(original equipment manufacturing) clearcoat materials normally used forthe original finishing of vehicles. The automobile industry is nowdemanding that this improved scratch resistance be transferred to theclearcoats used in the finishing of automobiles as well.

Replacing the OEM clearcoat materials or OEM powder slurry clearcoatmaterials commonly used in automotive finishing by sol-gel clearcoatmaterials is not possible, since the sol-gel clearcoats are too brittlefor this purpose, for example, and have poor optical properties(appearance). Above all, the sol-gel clearcoat materials are tooexpensive. The economically more favorable use of the sol-gel clearcoatmaterials as an additional coat over the clearcoats or powder slurryclearcoats used to date gives rise to adhesion problems between theclearcoat and the sol-gel coat, these problems arising in particularafter stone chipping and on exposure to condensation.

It is therefore an object of the present invention to provide a coatingsystem which comprises a clearcoat and which at the same time has verygood scratch resistance and adhesion, and a process for producing saidcoating system.

This object is surprisingly achieved by the process of the invention forproducing a coated substrate, in which a powder slurry clearcoatmaterial and, subsequently, a sol-gel clearcoat material are applied toa substrate to which, if desired, one or more coating compositions areapplied first of all, where

A) following application at a temperature above the film formationtemperature of the powder slurry clearcoat material and below thetemperature at which the crosslinking of this powder slurry clearcoatmaterial is complete, the powder slurry clearcoat material is predriedand induced to form a film,

B) subsequently the sol-gel clearcoat material is applied, and

C) finally both coatings are cured conjointly,

the powder slurry clearcoat material being only partly cured in step A)and the powder slurry clearcoat material and the sol-gel clearcoatmaterial being chosen such that a chemical bond between them ispossible.

It is found that automobile finishes produced by the process of theinvention have a very good scratch resistance which cannot be attainedusing the clearcoat systems that are commonly employed. Furthermore,relative to the prior art clearcoat/sol-gel clearcoat systems,outstanding adhesion of the coats was found, even on stone chipping orfollowing exposure to condensation, i.e., ten days' exposure of thecoats in an atmosphere of 40° C. and 100% relative humidity. The opticalproperties of the coating systems thus produced are also good, and nocracks are found in the scratch-resistant coating.

The process of the invention is elucidated in more detail below.

For the process of the invention it is possible to use any conceivablesubstrate. By way of example, mention may be made of substrates ofmetal, plastic, glass or ceramic. The substrate used with preference isa substrate of plastic or, in particular, of metal. The substrate usedin accordance with the invention may have any conceivable form, such asthe form of a vehicle body, especially an auto body. Furthermore, thesubstrate may also have been subjected to a surface treatment, such asgalvanizing or phosphating, for example.

In accordance with the process of the invention, the substrate may ifdesired be provided with one or more coats. The coating compositionsused for this purpose may comprise any coating compositions used inaccordance with the prior art. They may comprise, for example, liquidcoating compositions which are aqueous or contain organic solvents, orcoating compositions in powder form or in the form of a powder slurry.They may be applied by the processes known from the prior art, such asrolling, spraying, dipping, scattering, or by means of electrodepositioncoating, for example.

The process of the invention is preferably used in the context of themulticoat system customary for the finishing of automobiles. In thiscustomary multicoat system, for example, a primer, a surfacer coat, andalso basecoats, topcoats and clearcoats are applied to the automobilebody. The coating compositions used for the respective coats are knownto the skilled worker. The coats may be applied to the body in such away that, following the application of one coating film, it is driedand/or cured before the next film is applied, or two or more films areapplied by the technique known as wet-on-wet, in which these films aredried and/or cured conjointly.

In the context of the preferred use of the process of the invention,therefore, a multicoat system customary for automobile finishing iseffected, the clearcoat being formed from the powder slurryclearcoat/sol-gel clearcoat material used in accordance with theinvention.

The powder slurry clearcoat/sol-gel clearcoat system is preferably usedas a coating over basecoats, which are preferably part of a multicoatsystem, especially in the automobile industry. Particularly suitable asthe basecoat are aqueous basecoat materials based on a polyester,polyurethane resin and an amino resin. In this case the basecoatmaterials are normally subjected to partial drying, without beingcrosslinked, after which the powder slurry clearcoat film is appliedwet- on-wet.

The process of the invention is further used with preference for thecontinuous coating of metal sheets or metal strips by the coil coatingprocess. In this case, the powder slurry clearcoat/sol-gel clearcoatsystem may be applied directly to the metal surface or to a customaryand known primer coat that is present thereon. Not least, the process ofthe invention is also suitable for coating film-coated metal sheets andsubstrates of wood or glass. Accordingly, the process of the inventionmay be employed with advantage not only in the automobile sector butalso in the fields of industrial coating and of furniture coating.

Powder slurry coating materials are powder coating materials in the formof aqueous dispersions. Powder coating materials, i.e., coatingmaterials in powder form, were developed in order to replace the liquidcoating materials which are presently employed with preference for thecoating of automobile bodies which cause numerous environmentalproblems. The use of powder coating materials necessitates a differentapplication technology than the use of liquid coating materials.Therefore, powder coating materials in the form of aqueous dispersionswere developed which can be processed using the liquid coatingtechnology.

Powder slurry coating materials (also called powder coating slurries)are powder coating materials in the form of aqueous dispersions. Powderslurry clearcoat materials, accordingly, are powder clearcoat materialsin the form of aqueous dispersions.

In the text below, powder slurry coating materials always means powderslurry clearcoat materials, and powder coating materials always meanspowder clearcoat materials.

A powder slurry coating material normally consists of two components,namely the powder coating material I and an aqueous dispersion II. Thetwo components may not be brought together until they are on thesubstrate, e.g., by applying the powder coating material I to theoptionally coated substrate and then adding the aqueous dispersion II;preferably, the powder slurry coating material is formed by combiningthe two components prior to application to the substrate.

The first component of the powder slurry clearcoat material used inaccordance with the invention comprises a powder clearcoat material I.This may be any of the powder coating materials known to the skilledworker that are suitable for forming a clearcoat film.

In accordance with the invention it is of advantage if the powderclearcoat material I, in a first variant, comprises

a) at least one epoxide-containing binder containing from 0.5 to 40% byweight, based on the binder, of copolymerized glycidyl-containingmonomers, and

b) as crosslinking agent at least one polycarboxylic acid, in particulara straight-chain dicarboxylic acid, and/or a carboxy-functionalpolyester and also, if desired, at least onetris(alkoxycarbonylamino)triazine and/or at least one further customaryand known crosslinking agent

or alternatively

a) at least one oligomeric or polymeric, epoxide-containing crosslinkingagent containing from 0.5 to 40% by weight, based on the crosslinkingagent, of copolymerized glycidyl-containing monomers, and/or a lowmolecular mass, epoxide-containing crosslinking agent, and also, ifdesired at least one tris(alkoxycarbonylamino)triazine and/or at leastone further customary and known crosslinking agent; and

b) at least one carboxyl-containing polymer as binder,

both variants possibly comprising

c) at least one polyol.

The composition of the powder clearcoat material I may vary widely andmay be optimized to the particular end use. In accordance with theinvention it is of advantage if the powder clearcoat material I, basedon the respective solids, contains the constituents a), b) and c) in thefollowing amounts:

a) from 55 to 80, with particular preference from 60 to 78, and inparticular from 62 to 75% by weight,

b) from 14 to 30, with particular preference from 17 to 25, and inparticular from 18 to 23% by weight, and

c) from 0 to 22, with particular preference from 2 to 22, and inparticular from 4 to 20% by weight.

Suitable epoxy-functional binders a) for the powder clearcoat material Iare, for example, polyacrylate resins which contain epoxide groups andare preparable by copolymerizing at least one ethylenically unsaturatedmonomer containing at least one epoxide group in the molecule with atleast one further ethylenically unsaturated monomer containing noepoxide group in the molecule, at least one of the monomers being anester of acrylic acid or methacrylic acid. Polyacrylate resins of thiskind containing epoxide groups are known, for example, from the patentsEP-A-0 299 420, DE-B-22 14 650, DE-B-27 49 576, U.S. Pat. No. 4,091,048and U.S. Pat. No. 3,781,379.

Examples of suitable monomers for inventive use which contain no epoxidegroup in the molecule are alkyl esters of acrylic and methacrylic acid,especially methyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, n-butyl acrylate, n-butyl methacrylate, secondary-butylacrylate, secondary-butyl methacrylate, tert-butyl acrylate, tert-butylmethacrylate, neopentyl acrylate, neopentyl methacrylate, 2-ethylhexylacrylate or 2-ethylhexyl methacrylate; amides of acrylic acid andmethacrylic acid, especially acrylamide and methacrylamide;vinylaromatic compounds, especially styrene, methylstyrene orvinyltoluene; the nitriles of acrylic acid and methacrylic acid; vinylhalides and vinylidene halides, especially vinyl chloride or vinylidenefluoride; vinyl esters, especially vinyl acetate and vinyl propionate;vinyl ethers, especially n-butyl vinyl ether; or hydroxyl-containingmonomers, especially hydroxyethyl acrylate, hydroxyethyl methacrylate,hydroxypropyl acrylate, hydroxypropyl methacrylate, 4-hydroxybutylacrylate or 4-hydroxybutyl methacrylate.

Examples of suitable epoxy-functional monomers for inventive use areglycidyl acrylate, glycidyl meth-acrylate, or allyl glycidyl ether.

The polyacrylate resin containing epoxide groups normally has an epoxideequivalent weight of from 400 to 2500, preferably from 420 to 700, anumber-average molecular weight Mn (determined by gel permeationchromatography using a polystyrene standard) of from 2000 to 20000,preferably from 3000 to 10000, and a glass transition temperature Tg offrom 30 to 80, preferably from 40 to 70, with particular preference from40 to 60, and in particular from 48 to 52° C. (measured by means ofdifferential scanning calorimetry (DSC)).

The preparation of the polyacrylate resin containing epoxide groups hasno special features but instead takes place in accordance with thecustomary and known polymerization methods.

The further key constituent of the powder coating material I is thecrosslinking agent a) or b).

The crosslinking agent b) comprises carboxylic acids, especiallysaturated, straight-chain, aliphatic dicarboxylic acids having 3 to 20carbon atoms in the molecule. Instead of them or in addition to them itis also possible to use carboxy-functional polyesters. With veryparticular preference, dodecane-1,12-dicarboxylic acid is used.

In order to modify the properties of the powder coating materials I itis possible to use minor amounts of other carboxyl-containingcrosslinking agents b). Examples of suitable additional crosslinkingagents of this type are saturated branched or unsaturated straight-chaindicarboxylic and polycarboxylic acids and also the carboxyl-containingpolymers described below in detail as binders b).

Besides these carboxyl-containing crosslinking agents b), furthercrosslinking agents may be present.

In this context, the further crosslinking agents comprise, inparticular, tris(alkoxycarbonylamino)triazines and derivatives thereof.Examples of suitable tris(alkoxycarbonylamino)triazines are described inthe patents U.S. Pat. No. 4,939,213, U.S. Pat. No. 5,084,541 or EP-A-0624 577. Use is made in particular of tris(methoxy-, tris(butoxy- and/ortris(2-ethylhexoxycarbonylamino)triazines.

In accordance with the invention, preference is given to the methylbutyl mixed esters, the butyl 2-ethylhexyl mixed esters, and the butylesters. These have the advantage over the straight methyl ester ofbetter solubility in polymer melts.

The tris(alkoxycarbonylamino)triazines and their derivatives may also beused in a mixture with conventional crosslinking agents. Particularlysuitable for this purpose are blocked polyisocyanates or crosslinkerscontaining amino groups. It is also possible to use amino resins,examples being melamine resins. In this context it is possible to useany amino resin suitable for transparent topcoat materials or clearcoatmaterials, or a mixture of such amino resins.

In accordance with the invention, in a second variant, the powderclearcoat materials I may comprise an epoxy-functional crosslinkingagent a) and a carboxyl-containing binder b).

Examples of suitable carboxyl-containing binders b) for use inaccordance with the invention are, for example, polyacrylate resinsprepared by copolymerizing at least one ethylenically unsaturatedmonomer containing at least one acid group in the molecule with at leastone further ethylenically unsaturated monomer containing no acid groupsin the molecule.

Examples of highly suitable carboxyl-containing binders b) for use inaccordance with the invention are the polyacrylates andpolymethacrylates described below under number 1. and also numbers 1.1to 1.4, with a copolymerized acrylic acid and/or methacrylic acidcontent >0% by weight.

Examples of suitable oligomeric and polymeric epoxy-functionalcrosslinking agents a) for use in accordance with the invention are theabove-described binders a) containing epoxide groups.

Examples of suitable low molecular mass epoxy-functional crosslinkingagents a) for use in accordance with the invention are low molecularmass compounds containing at least two glycidyl groups, especiallypentaerythritol tetraglycidyl ether or triglycidyl isocyanurate.

Besides the epoxy-functional crosslinking agents a), the above-describedother crosslinking agents may be present.

The binder a) containing epoxide groups and the carboxyl-containingcrosslinking agent b) of the first variant of the invention, or thecarboxyl-containing binder b) and the epoxy-functional crosslinkingagent a) of the second variant of the invention, are generally used in aratio such that there are from 0.5 to 1.5, preferably from 0.75 to 1.25,equivalents of carboxyl groups per equivalent of epoxide groups. Theamount of carboxyl groups present may be determined simply by titrationwith an alcoholic KOH solution.

In accordance with the invention, the epoxy-functional binder a) or theoligomeric or polymeric, epoxy-functional crosslinking agent a) containsvinylaromatic compounds such as styrene in copolymerized form. In orderto limit the risk of cracking on weathering, the amount is, however, notmore than 35% by weight, based on the binder a) or the crosslinkingagent a). Preferably, from 10 to 25% by weight is incorporated bycopolymerization.

The powder clearcoat material I may comprise at least one polyol c).

Suitable polyols c) for use in accordance with the invention include alllow molecular mass compounds, oligomers and polymers which contain atleast two, preferably at least three, primary and/or secondary hydroxylgroups and which do not destroy the solid state of the powder clearcoatmaterial I.

Examples of suitable oligomers and polymers c) are linear and/orbranched and/or block, comb and/or random poly(meth)acrylates,polyesters, polyurethanes, acrylated polyurethanes, acrylatedpolyesters, polylactones, polycarbonates, polyethers,(meth)acrylate-diols, polyureas or oligomeric polyols.

Where these oligomers and polymers are used as polyols c), theypreferably contain no carboxyl groups.

These oligomers and polymers are known to the skilled worker, andnumerous suitable compounds are available on the market.

Of these oligomers and polymers c), the polyacrylates, the polyestersand/or the acrylated polyurethanes are of advantage and are thereforeused with preference.

Examples of particularly preferred oligomers and polymers c) for use inaccordance with the invention are

1. Polyacrylates having a hydroxyl number of from 40 to 240, preferablyfrom 60 to 210, in particular from 100 to 200, an acid number of from 0to 35, glass transition temperatures of from −35 to +85° C. and numberaverage molecular weights M_(n) of from 1500 to 300000.

The glass transition temperature of the polyacrylates is determined, asis known, by the nature and amount of the monomers used. The selectionof the monomers may be made by the skilled worker with the assistance ofthe following formula (A), in accordance with which the glass transitiontemperatures may be calculated approximately. $\begin{matrix}{{{{1/{Tg}} = {\sum\limits_{n = 1}^{n = x}\quad {W_{n}/{Tg}_{n}}}};{{\sum\limits_{n}\quad W_{n}} = 1}}{{Tg} = \text{glass transition temperature of the polyacrylate resin}}{W_{n} = {\text{weight fraction of the}\text{n}\text{-th monomer}}}{{Tg}_{n} = {\text{glass transition temperature of the homopolymer of the}\text{n}\text{-th monomer}}}{x = \text{number of different monomers.}}} & (A)\end{matrix}$

 Measures to control the molecular weight (e.g., selection ofappropriate polymerization initiators, use of chain transfer agents orof specific techniques of polymerization, etc.) are part of the art andneed not be elucidated further here.

1.1 Particularly preferred polyacrylates are preparable by polymerizing(al) from 10 to 92, preferably from 20 to 60% by weight of an alkylmethacrylate or cycloalkyl methacrylate having 1 to 18, preferably 4 to13 carbon atoms in the alkyl or cycloalkyl radical, or mixtures of suchmonomers, (a2) from 8 to 60, preferably from 12.5 to 50.0% by weight ofa hydroxyalkyl acrylate or a hydroxyalkyl methacrylate having 2 to 4carbon atoms in the hydroxyalkyl radical, or mixtures of such monomers,(a3) from 0 to 5, preferably from 0.7 to 3% by weight of acrylic acid ormethacrylic acid or mixtures of these monomers, and (a4) from 0 to 50,preferably up to 30% by weight of ethylenically unsaturated monomersdifferent than but copolymerizable with (a1), (a2) and (a3), or mixturesof such monomers, to give polyacrylates of the specification statedabove.

Examples of suitable (a1) components are methyl, ethyl, propyl, n-butyl,isobutyl, tert-butyl, pentyl, hexyl, heptyl or 2-ethylhexyl acrylate ormethacrylate and also cyclohexyl, tertbutylcyclohexyl or isobornylacrylate or methacrylate.

Examples of suitable (a2) components are hydroxyethyl, hydroxypropyl orhydroxybutyl or hydroxymethylcyclohexyl acrylate or methacrylate oradducts of (meth)acrylic acid and epoxides, such as Versatic acid®glycidyl esters.

Examples of suitable (a4) components are vinylaromatics such as styrene,vinyltoluene, alpha-methylstyrene, alpha-ethylstyrene, ring-substituteddiethylstyrenes, isopropylstyrene, butylstyrene and methoxystyrenes;vinyl ethers such as ethyl vinyl ether, n-propyl vinyl ether, isopropylvinyl ether, n-butyl vinyl ether or isobutyl vinyl ether; vinyl esterssuch as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pivalateor the vinyl ester of 2-methyl-2-ethylheptanoic acid; or allyl etherssuch as trimethylolpropane monoallyl, diallyl or triallyl ether, orethoxylated or propoxylated allyl alcohol.

1.2 Further examples of particularly preferred polyacrylates aredescribed in the European patent application EP-A-0 767 185 and in theAmerican patents U.S. Pat. Nos. 5,480,493, 5,475,073 or 5,534,598.

1.3 Further examples of particularly preferred polyacrylates are soldunder the brand name Joncryl®, such as, for instance, Joncryl® SCX 912and 922.5.

1.4 Further examples of particularly preferred polyacrylates are thoseobtainable by polymerizing (al) from 10 to 51% by weight, preferablyfrom 25 to 41% by weight, of 4-hydroxy-n-butyl acrylate or methacrylateor a mixture thereof, but especially 4-hydroxy-n-butyl acrylate, (a2)from 0 to 36% by weight, preferably from 0.1 to 20% by weight, of anon-(a1) hydroxyl-containing ester of acrylic acid or of methacrylicacid, or a mixture thereof, (a3) from 28 to 85% by weight, preferablyfrom 40 to 70% by weight, of a non-(a1) or -(a2) aliphatic orcycloaliphatic ester of methacrylic acid having at least four carbonatoms in the alcohol residue, or of a mixture of such monomers, (a4)from 0 to 3% by weight, preferably from 0.1 to 2% by weight, of anethylenically unsaturated carboxylic acid or a mixture of such acids,and (a5) from 0 to 20% by weight, preferably from 5 to 15% by weight, ofa non-(a1), -(a3) or -(a4) unsaturated monomer, or a mixture of suchmonomers, to give a polyacrylate having a hydroxyl number of from 60 to200, preferably from 100 to 160, an acid number of from 0 to 35, and anumber average molecular weight M_(n) of from 1500 to 10000, thecomposition of component (a3) being chosen such that polymerization ofthis component (a3) alone gives a polymethacrylate having a glasstransition temperature of from +10 to +100° C., preferably from +20 to+60° C.

Examples of suitable components (a2) are hydroxyalkyl esters of acrylicacid and methacrylic acid such as hydroxyethyl or hydroxypropyl acrylateor methacrylate, the choice being made such that polymerization of thiscomponent (a2) alone gives a polyacrylate having a glass transitiontemperature of from 0 to +80° C., preferably from +20 to +60° C.

Examples of suitable components (a3) are aliphatic esters of methacrylicacid having from four to 20 carbon atoms in the alcohol residue, such asn-butyl, isobutyl, tert-butyl, 2-ethylhexyl, stearyl and laurylmethacrylate; or cycloaliphatic esters of methacrylic acid, such ascyclohexyl methacrylate.

Examples of suitable components (a4) are acrylic acid and/or methacrylicacid.

Examples of suitable components (a5) are vinylaromatic hydrocarbons suchas styrene, alpha-alkylstyrene or vinyltoluene; amides of acrylic acidand methacrylic acid such as methacrylamide and acrylamide; nitrites ofacrylic acid and methacrylic acid; vinyl ethers or vinyl esters, thecomposition of this component (a5) preferably being made such thatpolymerization of components (a5) alone results in a polyacrylate havinga glass transition temperature of from +70 to +120° C., in particularfrom +80 to +100° C.

1.5 The preparation of these polyacrylates is widely known and isdescribed, for example, in the standard work Houben-Weyl, Methoden derorganischen Chemie, 4th edition, Volume 14/1, pages 24 to 255, 1961.

2. Polyester resins which are preparable by reacting (a1) at least onecycloaliphatic or aliphatic polycarboxylic acid, (a2) at least onealiphatic or cycloaliphatic polyol containing more than two hydroxylgroups in the molecule, (a3) at least one aliphatic or cycloaliphaticdiol, and (a4) at least one aliphatic, linear or branched saturatedmonocarboxylic acid, in a molar ratio of (a1):(a2):(a3):(a4)=1.0:0.2 to1.3:0.0 to 1.1:0.0 to 1.4, preferably 1.0:0.5 to 1.2:0.0 to 0.6:0.2 to0.9, to give a polyester or alkyd resin.

Examples of suitable components (a1) are hexahydrophthalic acid,1,4-cyclohexanedicarboxylic acid, endomethylenetetrahydrophthalic acid,oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid,pimelic acid, suberic acid, azelaic acid or sebacic acid.

Examples of suitable components (a2) are pentaerythritol,trimethylolpropane, triethylolethane and glycerol.

Examples of suitable components (a3) are ethylene glycol, diethyleneglycol, propylene glycol, neopentyl glycol,2-methyl-2-propyl-1,3-propanediol, 2-methyl-2-butyl-1,3-propanediol,2,2,4-trimethyl-1,5-pentanediol, 2,2,5-trimethyl-1,6-hexanediol,neopentyl glycol hydroxypivalate or dimethylolcyclohexane.

Examples of suitable components (a4) are 2-ethylhexanoic acid, lauricacid, isooctanoic acid, isononanoic acid or monocarboxylic acid mixturesobtained from coconut oil or palm kernel oil.

The preparation of the polyesters and alkyd resins used with preferencein accordance with the invention is widely known and is described, forexample, in the standard work Ullmanns Encyklopädie der technischenChemie, 3rd edition, Volume 14, Urban & Schwarzenberg, Munich, Berlin,1963, pages 80 to 89 and pages 99 to 105, and also in the followingbooks: “Résines Alkydes-Polyesters” by J. Bourry, Paris, Dunod, 1952,“Alkyd Resins” by C. R. Martens, Reinhold Publishing Corporation, NewYork, 1961, and “Alkyd Resin Technology” by T. C. Patton, IntersciencePublishers, 1962.

3. Polyurethanes as described in the patents EP-A-0 708 788, DE-A-44 01544 or DE-A-195 34 361.

Further examples of inventively suitable oligomers c) are oligomericpolyols which are obtainable by hydroformylation and subsequenthydrogenation from oligomeric intermediates obtained by metathesisreactions of acyclic monoolefins and cyclic monoolefins; examples ofsuitable cyclic monoolefins are cyclobutene, cyclopentene, cyclohexene,cyclooctene, cycloheptene, norbornene or 7-oxanorbornene; examples ofsuitable acyclic monoolefins are present in hydrocarbon mixtures whichare obtained in petroleum processing by cracking (C₅ cut); examples ofsuitable oligomeric polyols for use in accordance with the inventionhave a hydroxyl number (OHN) of from 200 to 450, a number averagemolecular weight Mn of from 400 to 1000, and a mass average molecularweight M_(w) of from 600 to 1100;

Examples of suitable low molecular mass compounds c) are branched,cyclic and/or acyclic C₉-C₁₆ alkanes functionalized with at least twohydroxyl groups, especially diethyloctanediols, and alsocyclohexanedimethanol, neopentyl glycol hydroxy-pivalate, neopentylglycol, trimethylolpropane or pentaerythritol.

Of the above-described polyols c), the carboxyl-free polyacrylates andpolymethacrylates which are described above under number 1.1 are of veryparticular advantage and are therefore used with very particularpreference.

The powder clearcoat materials I may comprise one or more suitablecatalysts for curing the epoxy resins. Examples of suitable catalystsare phosphonium salts and tetraalkylammonium salts of organic andinorganic acids, amines, imidazole and imidazole derivatives. Thecatalysts are used in general in amounts of from 0.001 to 2% by weight,based on the overall weight of the component a) or b) containing epoxidegroups, and also of the component b) or a) containing carboxyl groups.

Examples of suitable phosphonium salts are ethyltriphenylphosphoniumiodide, ethyltriphenylphosphonium chloride, ethyltriphenylphosphoniumthiocyanate, ethyltriphenylphosphonium acetate-acetic acid complex,tetrabutylphosphonium iodide, tetrabutylphosphonium bromide ortetrabutylphosphonium acetate-acetic acid complex. These and othersuitable phosphonium catalysts are described, for example, in thepatents U.S. Pat. No. 3,477,990 and U.S. Pat. No. 3,341,580.

Examples of suitable tetraalkylammonium salts are cetyltrimethylammoniumbromide and dicetyldimethylammonium bromide.

Examples of suitable imidazole catalysts are 2-styrylimidazole,1-benzyl-2-methylimidazole, 2-methylimidazole and 2-butylimidazole.These and other suitable imidazole catalysts are described in theBelgian patent no. 756 693.

The powder clearcoat material I may further comprise devolatilizers suchas benzoin, leveling agents based on polyacrylates, polysiloxanes orfluorine compounds, UV absorbers such as triazines and benzotriazoles,free-radical scavengers such as 2,2,6,6-tetramethylpiperidinederivatives, and/or antioxidants such as hydrazines, phosphoruscompounds as reducing agents, and 2,6-di-tert-butylphenol derivatives asfree-radical scavengers, and also further suitable auxiliaries andadditives.

The powder clearcoat slurry contains, based on its overall amount, thecomponent I in an amount of from 5 to 80, preferably from 10 to 70, withparticular preference from 15 to 60, and in particular from 20 to 50% byweight.

The component II consists substantially of water; advantageously,however, it is itself a dispersion comprising

IIa) at least one nonionic thickener and a dispersant and, if desired,

IIb) catalysts, auxiliaries, defoamers, preferably carboxy-functionaldispersants, wetting agents, antioxidants, UV absorbers, free-radicalscavengers, small amounts of solvents, leveling agents, biocides,crosslinking agents and/or water retention agents.

In accordance with the invention the component II contains, based on itsoverall amount,

from 0.01 to 20, preferably from 1 to 15, with particular preferencefrom 2 to 10, and in particular from 5 to 9% by weight of component IIa)and also

from 0.001 to 20, preferably from 0.01 to 15, with particular preferencefrom 0.1 to 10, and in particular from 1 to 9% by weight of componentIIb).

The suitable nonionic associative thickeners have the followingstructural features:

IIaa) a hydrophilic structure which ensures sufficient solubility inwater, and

IIab) hydrophobic groups capable of associative interaction in theaqueous medium.

Examples of suitable hydrophobic groups IIab) are long-chain alkylradicals such as dodecyl, hexadecyl, or octadecyl radicals or alkylarylradicals such as octylphenyl or nonylphenyl radicals.

Examples of suitable hydrophilic structures IIaa) are polyacrylates,cellulose ethers or, in particular, polyurethanes, which contain thehydrophilic groups as polymer building blocks. Particularly preferredhydrophilic structures in this context are polyurethanes containingpolyether chains, preferably from polyethylene oxide, as buildingblocks.

Preferred dispersants IIa) used are polyurethane resins.

The polyurethane resins IIa) employed with preference consist preferablyof

1. at least one organic component having at least two reactive hydrogenatoms,

2. a monofunctional ether, and

3. a polyisocyanate.

The organic component of the polyurethane composition IIa) comprises apolyester polyol, a low molecular mass diol and/or triol, or mixturesthereof. If desired, a trifunctional hydroxyl-containing monomer can beused as well.

In a second preferred embodiment the polyurethane IIa) comprises

1. at least one organic component having at least two reactive hydrogenatoms,

2. a nonionic stabilizer which is prepared by reacting

i. a monofunctional polyether with a polyisocyanate component to producean isocyanate intermediate and

ii. a component having at least one active amine group and at least twoactive hydroxyl groups, and

3. at least one polyisocyanate component.

The organic component preferably comprises a polyether polyester polyol,a low molecular mass diol and/or triol, or mixtures thereof.

The polyester component can be prepared by reacting at least onedicarboxylic acid and at least one alcohol component, the alcoholcomprising at least two hydroxyl groups. The carboxylic acid componentcomprises two or more carboxyl groups.

In addition to the carboxylic acids the polyester resin can alsocomprise one or more low molecular mass diols or triols. In principle,any polyol can be employed.

The polyester resins or mixtures of polyester resins employed comprisepreferably terminal hydroxyl groups. This is brought about by adding anexcess of polyols.

For the synthesis of the polyesters it is possible to employ bothmonocarboxylic acids and monoalcohols. Preferably, however, themonocarboxylic acids and/or monoalcohols are present in a very smallamount by weight in the polyester resin.

The polyester diol components employed with preference comprise between20 and 80% by weight of the polyurethane resin IIa). The amounts arepreferably between 50 and 70% by weight. Very particular preference isgiven to from 55 to 65% by weight.

The polyurethane IIa) is prepared employing polyester polyols having amolecular weight of between 500 and 5000. Preference is given tomolecular weights of between 1000 and 3500.

In addition to the polyester diols, the polyurethane resins IIa) cancomprise further organic components having at least two reactivehydrogen atoms. These are preferably diols and triols, thiols and/oramines, or mixtures of these substances. The components used tosynthesize the polyester component can also be employed here as separatecomponents. In other words, dialcohols or trialcohols, such as neopentylglycol or 1,6-hexanediol, for example, are also suitable as anadditional organic component in the polyurethane IIa).

The molecular weight of the diols and/or triols employed in thepolyurethane resin IIa) is between 0 and 20% by weight. Preference isgiven to from 1 to 6% by weight.

The polyurethane resin IIa) additionally comprises polyisocyanates,especially diisocyanates. The isocyanates are present at between 5 and40% by weight based on the polyurethane mass. Particular preference isgiven to from 10 to 30% by weight and very particular preference to from10 to 20% by weight. To prepare the polyurethane IIa), finally, amonofunctional polyether is employed.

In a second variant, a nonionic dispersant IIa) is prepared by,preferably, reacting a monofunctional polyether with a diisocyanate. Theresultant reaction product is then reacted with a component whichcomprises at least one active amine group and at least two activehydroxyl groups.

In one particular embodiment the polyurethane IIa) comprises a reactionproduct of:

1. a polyester polyol, which is in turn a reaction product of acarboxylic acid having at least two carboxyl groups and a componenthaving at least two hydroxyl groups,

2. at least one low molecular mass component having at least twohydroxyl groups,

3. at least one polyisocyanate component,

4. a nonionic stabilizer prepared by reacting a monofunctional etherwith a polyisocyanate and then reacting the resultant reaction productwith a component which comprises at least one active amine group and atleast two active hydroxyl groups.

In a fourth variant, the polyurethane comprises a reaction product of

1. a polyester polyol,

2. at least one low molecular mass diol or triol,

3. a polyisocyanate,

4. a trihydroxyl-containing monomer,

5. a monofunctional hydroxyl-containing polyether.

The polyesters are synthesized using the above-described carboxylic acidcomponents and an excess of polyols. The excess of polyols is chosen sothat preferably terminal hydroxyl groups are formed. The polyolspreferably have a hydroxyl functionality of at least two.

The polyester resin consists preferably of one or more polyols,preferably comprising one diol. Diols employed with preference arealkylene glycols, such as ethylene glycol, propylene glycol, butyleneglycol and neopentyl glycol, 1,6-hexanediol or other glycols, such asbisphenol A, cyclohexanedimethanol, caprolactonediol, hydroxyalkylatedbisphenol, and similar compounds.

The low molecular mass diols which are preferably employed in accordancewith the invention are known from the prior art. They include aliphaticdiols, preferably alkylene polyols having 2 to 18 carbon atoms. Examplesof such are 1,4-butanediol and cycloaliphatic diols, such as1,2-cyclohexanediol and cyclohexanedimethanol.

Suitable organic polyisocyanates in accordance with the invention arepreferably those which comprise at least two isocyanate groups.Particular preference is given to diisocyanates, e.g. p-phenylenediisocyanate, biphenyl 4,4′-diisocyanate, toluene diisocyanates (TDI),3,3′-dimethyl-4,4′-biphenylene diisocyanate, 1,4-tetramethylenediisocyanate, 1,6-hexamethylene diisocyanate (HDI),2,2,4-trimethylhexane 1,6-diisocyanate, methylenebis(phenylisocyanates), 1,5-naphthalene diisocyanate, bis(isocyanatoethylfumarate), isophorone diisocyanate (IPDI) and methylenebis(4-cyclohexylisocyanate).

In addition to the abovementioned diisocyanates, other multifunctionalisocyanates are also used. Examples are 1,2,4-benzene triisocyanate andpolymethylenepolyphenylene polyisocyanate.

Particular preference is given to the use of aliphatic diisocyanates,examples being 1,6-hexamethylene diisocyanate, 1,4-butylenediisocyanate, methylenebis(4-cyclohexyl isocyanate), isophoronediisocyanate and 2,4-toluene diisocyanate.

Relatively long-chain polyurethane resins IIa) can be obtained by chainextension with components containing diol and/or triol groups.Particular preference is given to chain extenders having at least tworeactive hydrogen atoms, examples being diols, thiols, diamines,alkanolamines, aminoalkyl mercaptans or hydroxyalkyl mercaptans ormixtures of these substances.

Examples of diols employed as chain extenders are 1,6-hexanediol,cyclohexanedimethylol and 1,4-butanediol. A particularly preferred diolis neopentyl glycol.

The polyethers inventively employed are preferably mono- or difunctionalpolyethers. The monofunctional ones include, for example, those preparedby polymerizing ethylene oxides, propylene oxides or mixtures thereof.

The polyurethane IIa) described can be mixed with conventionalcrosslinkers. These include preferably amino resins, e.g. melamine. Itis also possible to employ condensation products of other amines andamides, e.g. aldehyde condensates of triazines, diazines, triazoles,guanadines, guanamines or alkyl- and aryl-substituted derivatives ofsuch components. Some examples of such components are N,N′-dimethylurea,dicyandiamide, 2-chloro-4,6-diamino-1,3,5-triazine,6-methyl-2,4-diamino-1,3,5-triazine, 3,5-diaminotriazole,triaminopyrimidines, 2-mercapto-4,6-diaminopyrimidine,2,4,6-triethyltriamino-1,3,5-triazine and similar substances.

The preferred aldehyde comprises formaldehyde. It is also possible toemploy acetaldehyde, crotonaldehyde, acrolein, benzaldehyde andfurfural.

The amine-aldehyde condensation products can comprise methylol orsimilar alcohol groups. Examples of alcohols which can be employed aremethanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol,benzyl alcohol and aromatic alcohols, cyclic alcohols, such ascyclohexanol, monoethers or glycols, and also substituted alcohols,e.g., 3-chloropropanol.

In addition to the abovementioned isocyanates it is also possible toemploy blocked polyisocyanates as crosslinking agents. Examples of theseinclude organic polyisocyanates such as trimethylene, tetramethylene,hexamethylene, 1,2-propylene, 1,2-butylene and 2,3-butylenediisocyanate. Further candidates for use include cycloalkane componentssuch as 1,3-cyclopentane, 1,4-cyclohexane and 1,2-cyclohexanediisocyanates. It is also possible to use aromatic components such asphenylene, p-phenylene, 4,4′-diphenyl, 1,5-napthalene and1,4-naphthalene diisocyanates. Components suitable in addition to theseare aliphatic-aromatic components such as 4,4′-diphenylenemethane, 2,4-or 2,6-tolylene, or mixtures thereof, 4,4′-toluidine and 1,4-xylylenediisocyanates. Further examples are ring-substituted aromatic componentssuch as 4,4′-diphenyl ether diisocyanate and chlorodiphenylenediisocyanates. Triisocyanates which can be employed are triphenylmethane4,4′,4″-triisocyanate, 1,3,5-triisocyanatobenzene and2,4,6-triisocyanatotoluene. A tetraisocyanate which can be used,finally, is 4,4′-diphenyldimethylmethane 2,2′,5,5′-tetraisocyanate.

Blocking agents which can be employed include aliphatic, cycloaliphaticand aromatic alkyl monoalcohols. Examples of these include methyl,ethyl, chloroethyl, propyl, butyl, cyclohexyl, heptyl, octyl, nonyl,3,3,5-trimethylhexanol, decyl and lauryl alcohols. Examples of phenoliccomponents which can be used are phenols or substituted phenols.Examples thereof are cresol, xylenol, nitrophenol, chlorophenol,ethylphenol, 1-butylphenol and 2,5-di-tert-butyl-4-hydroxytoluene.

Further suitable blocking agents are tertiary hydroxylamines, e.g.,diethylethanolamine, and oximes, such as methyl ethyl ketone oxime,acetone oxime and cyclohexanone oxime.

In the process of the invention, moreover, particular preference isgiven to the use of the powder clearcoat slurries of the Germanapplication 198 14 471.7, unpublished at the priority date of thepresent specification; they are hereby introduced into the process ofthe invention.

The solid powder coating materials I are prepared in accordance with theknown methods (cf., e.g., BASF Lacke+Farben AG Product InformationBulletin “Pulverlacke” [Powder Coating Materials], 1990) by homogenizingand dispersing, for example by means of an extruder, screw compounder,and the like. Following preparation of the powder coating materials,they are prepared for dispersing by grinding and, if appropriate, byclassifying and sieving.

Finally, the aqueous powder slurry clearcoat material or the powderclearcoat material dispersion may be obtained from the powder coatingmaterial I as first component and from the aqueous dispersion II assecond component, by wet grinding or by stirred incorporation ofdry-ground powder coating material I into the aqueous dispersion II. Wetgrinding is preferred.

Following the dispersion of component I in component II, grinding takesplace, if desired, the pH is adjusted to 4.0 to 7.0, preferably 5.5 to6.5 and the resulting slurry is filtered.

The average particle size is, for example, between 1 and 25 μm,preferably below 20 μm, and with particular preference between 3 and 10μm.

Before or after the wet grinding or the introduction of the dry powdercoating material I into water, possibly already containing constituentsof the aqueous dispersion, there may be added to the dispersion II adefoamer mixture, an ammonium salt and/or alkali metal salt, apolyurethane-based dispersant IIa), a carboxy-functional dispersant,wetting agent and/or thickener mixture, and also the other additives.Preferably, defoamer, dispersant, wetting agent and/or thickener arefirst dispersed in water, after which the powder clearcoat material I isstirred in, in small portions. Subsequently, defoamer, dispersant,wetting agent and/or thickener are dispersed in again. Finally, thepowder clearcoat material I is stirred in again, in small portions.

In accordance with the invention the pH is preferably adjusted usingammonia and/or amines. The pH may initially rise, giving a stronglybasic dispersion.

However, the pH falls back to the above-indicated levels within a fewhours or days.

The powder slurry clearcoat material may be applied to the uncoated orcoated substrate using the methods which are known from liquid coatingtechnology (e.g., spraying, rolling, or dipping). In particular, it isapplied by means of spraying techniques.

For coatings, it is common to use coating materials based on organicpolymers. By organic polymers are meant those formed essentially fromcarbon-containing monomers, the carbon atoms being incorporated into theresultant polymer chain. The powder slurry clearcoat material is also,in this sense, a coating material based on organic polymers.

In comparison, the so-called sol-gel clearcoat materials which are keyto the process of the invention comprise siloxane-containing coatingformulations which can be prepared by reacting hydrolyzable siliconcompounds with water or water donors and which comprise organicconstituents in order to improve certain properties. A generaldescription of such systems is given, for example, in the article byBruce M. Novak, “Hybrid Nanocomposite Materials-Between InorganicGlasses and Organic Polymers”, in Advanced Materials, 1993, 5, No. 6,pp. 422-433, or in the presentation by R. Kasemann, H. Schmidt, 15thInternational Conference, International Centre for Coatings Technology,Paper 7, “Coatings for mechanical and chemical protection based onorganic-inorganic sol-gel nanocomposites”, 1993.

The base reactions take place in a sol-gel process in whichtetraorthosilicates are hydrolyzed and condensed in the presence orabsence of a cosolvent:

Hydrolysis

Si(OR)₄+H₂O→(RO)₃Si—OH+ROH

Condensation

—Si—OH+HO—Si—→—Si—O—Si—+H₂O

—Si—OH+RO—Si—→—Si—O—Si—+ROH

where R can be an alkyl group, such as methyl or ethyl. Frequently,tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) areused. To catalyze the reactions, acids, bases or fluoride ions are used.

The sol-gel clearcoat materials used in accordance with the inventionare siloxane-containing structures modified with organic constituents(ormocer=organically modified ceramic).

By means of controlled hydrolysis and condensation of silicic estersand, if appropriate, of metal alkoxides, base materials for coatings areprepared. The systems acquire specific properties through theincorporation of organically modified silicic acid derivatives into thesilicatic network. They permit the construction of an organic polymernetwork in addition to the inorganic framework, if polymerizable organicradicals (e.g., olefins, epoxides) are used.

The modification can come about, for example, as a result of thepresence of a ready-made organic polymer during the hydrolysis andcondensation of the starting materials or in the sol (type I).

If the polymer present contains functional groups, such astrialkoxysilyl units, —CH₂Si(OR)₃, for example, which are able to reactwith the inorganic phase, a material is obtained which contains covalentbonds between the inorganic and the organic phase (type II).

Further, the organically modified sol-gel systems are obtained bysimultaneous polymerization of the inorganic and organic phase (typeIII).

With these types as well, chemical bonds may be obtained between theinorganic and the organic phase as a result of appropriate functionalgroups (type IV).

Alternatively, it is possible to use sol-gel clearcoat materialsobtained by incorporating organically modified silicic acid derivativesinto the silicatic network without adding polymerizable organicradicals.

The sol-gel clearcoat materials used in accordance with the inventioncan be obtained, for example, by controlled hydrolysis and condensationof organically modified hydrolyzable silane. This can be done, ifdesired, in the presence of organic monomers, solvents, organicallymodified hydrolyzable metal alkoxides, and metal oxides in the form ofnanoparticles.

The hydrolyzable silane comprises compounds of the general formula (B)

SiR₄  (B)

in which the radicals R, which may be identical or different, areselected from hydrolyzable groups, hydroxyl groups, and nonhydrolyzablegroups.

The nonhydrolyzable groups R in the general formula (B) are preferablyselected from alkyl, having in particular 1 to 4 carbon atoms, such asmethyl, ethyl, propyl and butyl; alkenyl, having in particular 2 to 4carbon atoms, such as vinyl, 1-propenyl, 2-propenyl and butenyl;alkynyl, having in particular 2 to 4 carbon atoms, such as acetylenyland propargyl; and aryl, having in particular 6 to 10 carbon atoms, suchas phenyl and naphthyl, for example. Nonhydrolyzable groups R used arepreferably alkyl groups.

Examples of hydrolyzable groups R in the above formula (B) are alkoxy,having in particular 1 to 4 carbon atoms, such as methoxy, ethoxy,n-propoxy, isopropoxy and butoxy; aryloxy, having in particular 6 to 10carbon atoms, such as phenoxy; acyloxy, having in particular 1 to 4carbon atoms, such as acetoxy and propionyloxy; and alkylcarbonyl, suchas acetyl, for example.

In addition to the abovementioned preferred hydrolyzable groups R, thefollowing may be mentioned as further groups which are likewisesuitable: hydrogen and alkoxy radicals having 5 to 20, especially 5 to10 carbon atoms and alkoxy-substituted alkoxy groups, such asbeta-methoxyethoxy, for example. Particularly preferred hydrolyzablegroups R are those which carry no substituents and lead to hydrolysisproducts of low molecular weight, examples being lower alcohols, such asmethanol, ethanol, propanol, n-butanol, isobutanol, sec-butanol, andtert-butanol.

At least one group R of the formula (B) must be a hydrolyzable group;compounds of the formula (B) having three or four hydrolyzable groups Rare particularly preferred.

In addition, the hydrolyzable silanes preferably include anonhydrolyzable group R which contains a functional group. Thesefunctional groups can, for example, be epoxy groups, amino groups,olefinically unsaturated groups such as vinyl or (meth)acrylic groups,mercapto groups, isocyanate groups and/or reaction products thereof withfurther reactive compounds.

Examples of highly suitable hydrolyzable silanes for use in accordancewith the invention are methyltriethoxysilane, methyltrimethoxysilane,tetramethyl orthosilicate, tetraethyl orthosilicate,3-glycidyloxypropyltrimethoxysilane, and 3-aminopropyltriethoxysilane.

The compounds of the general formula (B) can be used in whole or in partin the form of precondensates, i.e., compounds formed by partialhydrolysis of the compounds of the formula (B), either alone or in amixture with other hydrolyzable compounds.

Organic monomers which can be used are all monomers known to the skilledworker for the formation of polymers.

Examples of suitable monomers are the monomers described above which areused to prepare polyacrylates, polyesters or polyurethanes, and themonomers described below which are used to prepare the acryliccopolymers (A1).

Where the mixture includes organic monomers, the hydrolysis andcondensation of the hydrolyzable silanes (B) is preferably conducted insuch a way that the corresponding polymers are formed from the organicmonomers. For this purpose it is possible to use the customary and knowninitiators, examples being those described below (Type III).

The hydrolysis and condensation of the hydrolyzable silanes (B), or themodification of the sol-gel clearcoat material, is preferably conductedin the presence of a ready-made organic polymer (Type I).

This polymer is preferably used as a solution in organic solvents.

Particular preference is given to the use of acrylic copolymers (A1)prepared by copolymerizing the following monomers (a11) and (a13) andalso, if desired, further monomers (a12), (a14), (a15) and/or (a16), thenature and amount of (a11) and (a13) and also, if used, (a12), (a14),(a15) and (a16) being selected such that the acrylic copolymer (Al) hasthe desired OH number, acid number, and the desired molecular weight.Preferably, the acrylic copolymers (A1) have a hydroxyl number of from 0to 240, with particular preference from 0 to 200, and in particular from0 to 150, an acid number of from 5 to 100, with particular preferencefrom 10 to 60, and in particular from 20 to 40, glass transitiontemperatures of from −35 to +85° C., and number-average molecularweights Mn of from 1500 to 300000.

The polyacrylate resins used inventively may be prepared using asmonomer (a11) any (meth)acrylic acid alkyl or cycloalkyl ester which iscopolymerizable with (a12), (a13), (a14), (a15) and (a16) and has up to20 carbon atoms in the alkyl radical, especially methyl, ethyl, propyl,n-butyl, sec-butyl, tert-butyl, hexyl, ethylhexyl, stearyl and laurylacrylate or methacrylate; cycloaliphatic (meth)acrylates, especiallycyclohexyl, isobornyl, dicyclopentadienyl,octahydro-4,7-methano-1H-indenemethanol or tertbutylcyclohexyl(meth)acrylate; (meth)acrylic oxaalkyl or oxacycloalkyl esters such asethyltriglycol (meth)acrylate and methoxyoligoglycol (meth)acrylatehaving a molecular weight Mn of preferably 550; or other ethoxylatedand/or propoxylated, hydroxyl-free (meth)acrylic acid derivatives. Thesemonomers may include, in minor amounts, more highly functional(meth)acrylic alkyl or cycloalkyl esters such as ethylene glycol,propylene glycol, diethylene glycol, dipropylene glycol, butyleneglycol, 1,5-pentanediol, 1,6-hexanediol,octahydro-4,7-methano-1H-indenedimethanol or 1,2-, 1,3- or1,4-cyclohexanediol di(meth)acrylate; trimethylolpropane di- ortri(meth)acrylate; or pentaerythritol di-, tri- or tetra(meth)acrylate.In the context of the present invention, minor amounts ofhigher-functional monomers are those amounts that do not lead tocrosslinking or gelling of the polyacrylate resins.

As the monomer (a12) it is possible to use any ethylenically unsaturatedmonomers which are copolymerizable with (a11), (a12), (a13), (a14),(a15) and (a16) and different from (a15) which carry at least onehydroxyl group per molecule and are essentially free from acid groups,such as hydroxyalkyl esters of acrylic acid, methacrylic acid or anotheralpha,beta-ethylenically unsaturated carboxylic acid which are derivedfrom an alkylene glycol which is esterified with the acid or areobtainable by reacting the acid with an alkylene oxide; especiallyhydroxyalkyl esters of acrylic acid, methacrylic acid, ethacrylic acid,crotonic acid, maleic acid, fumaric acid or itaconic acid, in which thehydroxyalkyl group contains up to 20 carbon atoms, such as2-hydroxyethyl, 2-hydroxypropyl, 3-hydroxypropyl, 3-hydroxybutyl,4-hydroxybutyl acrylate, methacrylate, ethacrylate, crotonate, maleate,fumarate or itaconate; 1,4-bis(hydroxymethyl)cyclohexane,octahydro-4,7-methano-1H-indenedimethanol or methylpropanediolmonoacrylate, monomethacrylate, monoethacrylate, monocrotonate,monomaleate, monofumarate or monoitaconate; or reaction products ofcyclic esters, such as epsiloncaprolactone, for example, and thesehydroxyalkyl esters; or olefinically unsaturated alcohols such as allylalcohol or polyols such as trimethylolpropane monoallyl or diallyl etheror pentaerythritol monoallyl, diallyl or triallyl ether. As far as thesehigher-functional monomers (a12) are concerned, the comments made forthe higher-functional monomers (a11) apply analogously. The proportionof trimethylolpropane monoallyl ether is usually from 2 to 10% byweight, based on the overall weight of the monomers (a11) to (a16) usedto prepare the polyacrylate resin. In addition, however, it is alsopossible to add from 2 to 10% by weight, based on the overall weight ofthe monomers used to prepare the polyacrylate resin, oftrimethylolpropane monoallyl ether to the finished polyacrylate resin.The olefinically unsaturated polyols, such as trimethylolpropanemonoallyl ether in particular, may be used as sole hydroxyl-containingmonomers, but in particular may be used proportionately in combinationwith other of the abovementioned hydroxyl-containing monomers.

As the monomer (a13) it is possible to use any ethylenically unsaturatedmonomer, or mixture of such monomers, which carries at least one acidgroup, preferably one carboxyl group, per molecule and iscopolymerizable with (a11), (a12), (a14), (a15) and (a16). As component(a13) it is particularly preferred to use acrylic acid and/ormethacrylic acid. However, other ethylenically unsaturated carboxylicacids having up to 6 carbon atoms in the molecule may also be used.Examples of such acids are ethacrylic acid, crotonic acid, maleic acid,fumaric acid, and itaconic acid. It is further possible to useethylenically unsaturated sulfonic or phosphonic acids, and/or theirpartial esters, as component (a13). Further suitable components (a13)include mono(meth)acryloyloxyethyl maleate, succinate and phthalate.

As the monomer (a14) it is possible to use one or more vinyl esters ofalpha-branched monocarboxylic acids having 5 to 18 carbon atoms in themolecule. The branched monocarboxylic acids may be obtained by reactingformic acid or carbon monoxide and water with olefins in the presence ofa liquid, strongly acidic catalyst; the olefins may be cracking productsof paraffinic hydrocarbons, such as mineral oil fractions, and maycomprise branched and straight-chain acyclic and/or cycloaliphaticolefins. The reaction of such olefins with formic acid or with carbonmonoxide and water produces a mixture of carboxylic acids in which thecarboxyl groups are located predominantly on a quaternary carbon atom.Other olefinic starting materials are, for example, propylene trimer,propylene tetramer and diisobutylene. Alternatively, the vinyl estersmay be prepared in a conventional manner from the acids; for example, byreacting the acid with acetylene. Particular preference, owing to theirready availability, is given to the use of vinyl esters of saturatedaliphatic monocarboxylic acids having 9 to 11 carbon atoms which arebranched on the alpha carbon atom.

As the monomer (a15), use is made of the reaction product of acrylicacid and/or methacrylic acid with the glycidyl ester of analpha-branched monocarboxylic acid having 5 to 18 carbon atoms permolecule. Glycidyl esters of highly branched monocarboxylic acids areavailable under the trade name “Cardura”. The reaction of the acrylic ormethacrylic acid with the glycidyl ester of a carboxylic acid having atertiary alpha carbon atom can take place before, during or after thepolymerization reaction. As the component (a15) it is preferred to usethe reaction product of acrylic acid and/or methacrylic acid with theglycidyl ester of Versatic acid. This glycidyl ester is commerciallyavailable under the name “Cardura E10”.

As the monomer (a16) it is possible to use all ethylenically unsaturatedmonomers, or mixtures of such monomers, which are copolymerizable with(a11), (a12), (a13), (a14) and (a15), are different from (a11), (a12),(a13) and (a14), and are substantially free from acid groups. Suitablecomponents (a16) include the following:

olefins such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene,cyclohexene, cyclopentene, norbornene, butadiene, isoprene,cyclopentadiene and/or dicyclopentadiene;

(meth)acrylamides such as (meth)acrylamide, N-methyl-, N,N-dimethyl-,N-ethyl-, N,N-diethyl-, N-propyl-, N,N-dipropyl-, N-butyl-,N,N-dibutyl-, N-cyclohexyl- and/or N,N-cyclohexylmethyl(meth)acrylamide;

monomers containing epoxide groups, such as the glycidyl ester ofacrylic acid, methacrylic acid, ethacrylic acid, crotonic acid, maleicacid, fumaric acid and/or itaconic acid;

vinylaromatic hydrocarbons, such as styrene, alpha-alkylstyrenes,especially alpha-methylstyrene, and/or vinyltoluene;

nitrites such as acrylonitrile and/or methacrylonitrile;

vinyl compounds such as vinyl chloride, vinyl fluoride, vinylidenedichloride, vinylidene difluoride; N-vinylpyrrolidone; vinyl ethers suchas ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether,n-butyl vinyl ether, isobutyl vinyl ether and/or vinyl cyclohexyl ether;vinyl esters such as vinyl acetate, vinyl propionate, vinyl butylate,vinyl pivalate and/or the vinyl ester of 2-methyl-2-ethylheptanoic acid;and/or

polysiloxane macromonomers having a number-average molecular weight Mnof from 1000 to 40000, preferably from 2000 to 20000, with particularpreference from 2500 to 10000 and in particular from 3000 to 7000 andhaving on average from 0.5 to 2.5, preferably from 0.5 to 1.5,ethylenically unsaturated double bonds per molecule, as described in DE38 07 571 A1 on pages 5 to 7, in DE 37 06 095 A1 in columns 3 to 7, inEP 0 358 153 B1 on pages 3 to 6, in U.S. Pat. No. 4,754,014 in columns 5to 9, in DE 44 21 823 A1 or in the international patent application WO92/22615 on page 12, line 18 to page 18, line 10, oracryloxysilane-containing vinyl monomers, preparable by reactinghydroxy-functional silanes with epichlorohydrin and subsequentlyreacting the reaction product with methacrylic acid and/or hydroxyalkylesters of (meth)acrylic acid.

Preference is given to using vinylaromatic hydrocarbons, especiallystyrene.

The nature and amount of the monomers (a11) to (a16) is selected suchthat the acrylic copolymer (A1) has the desired OH number, acid number,and glass transition temperature. Acrylic copolymers (A1) used withparticular preference are obtained by polymerizing

(a11) from 20 to 99% by weight, preferably from 30 to 50% by weight, ofthe component (a11),

(a12) from 0 to 50% by weight, preferably from 0 to 40% by weight, ofthe component (a12),

(a13) from 1to 20% by weight, preferably from 1to 10% by w eight, of thecomponent (a13),

(a14) from 0 to 25% by weight of the component (a14),

(a15) from 0 to 25% by weight of the component (a15), and

(a16) from 0 to 30% by weight, preferably from 10 to 20% by weight, ofthe component (a16),

the sum of the weight fractions of the components (a11) to (a16) being100% in each case.

The inventively employed acrylic copolymers (A1) are prepared in anorganic solvent or solvent mixture, which is preferably free fromaromatic solvents, and in the presence of at least one polymerizationinitiator. Polymerization initiators used are the polymerizationinitiators which are customary for the preparation of acryliccopolymers.

Examples of suitable polymerization initiators are initiators which formfree radicals, such as, for example, tert-butyl peroxyethylhexanoate,benzoyl peroxide, di-tert-amyl peroxide, azobisisobutyronitrile, andtert-butyl perbenzoate. The initiators are used preferably in an amountof from 1to 25% by weight, with particular preference from 2 to 10% byweight, based on the overall weight of the monomers.

The polymerization is judiciously conducted at a temperature of from 80to 200° C., preferably from 110 to 180° C.

Preferred solvents used are ethyl glycol, ethoxyethyl propionate andisopropoxypropanol.

The acrylic copolymer (A1) may be prepared by a two-stage process ifmonomers additional to the mandatory monomers (a11) and (a13) are used.This is done by

1. polymerizing a mixture of the monomers (a11) and (a12) and, ifdesired, (a14, (a15) and/or (a16), or a mixture of portions of themonomers (a11) and (a12) and also, if desired, (a14), (a15) and/or(a16), in an organic solvent, and

2. after at least 60% by weight of the mixture of (a11) and (a12) and,if desired, (a14), (a15) and/or (a16) have been added, adding themonomer (a13) and any remainder of the monomers (a11) and (a12) and, ifappropriate, (a14), (a15) and/or (a16), and continuing polymerization.

In addition, however, it is also possible to include the monomers (a14)and/or (a15) in the initial charge, together with at least some of thesolvent, and to meter in the remaining monomers. Furthermore, it is alsopossible for only some of the monomers (a14) and/or (a15) to be includedin the initial charge, together with at least some of the solvent, andfor the remainder of these monomers to be added as described above.Preferably, for example, at least 20% by weight of the solvent and about10% by weight of the monomers (a14) and (a15), and, if desired, portionsof the monomers (a11) and (a16), are included in the initial charge.

Preference is further given to a two-stage process in which the firststage lasts for from 1to 8 hours, preferably from 1.5 to 4 hours, andthe mixture of (a13) and any remainder of the monomers (a11), (a12) and,if appropriate, (a14), (a15) and (a16) is added over the course of from20 to 120 minutes, preferably over the course of from 30 to 90 minutes.Following the end of the addition of the mixture of (a13) and anyremainder of the monomers (a11) and (a12) and, if appropriate, (a14),(a15) and (a16) polymerization is continued until all of the monomersused have undergone essentially complete reaction. In this case, thesecond stage may follow on immediately from the first. Alternatively,the second stage may be commenced only after a certain time, forexample, after from 10 minutes to 10 hours.

The amount, and rate of addition, of the initiator is preferably chosenso as to give an acrylic copolymer (A1) having a number-averagemolecular weight Mn of from 1000 to 30000 daltons. It is preferred tocommence the addition of initiator some time, generally from about 1to15 minutes, before the addition of the monomers. Furthermore, preferenceis given to a process in which the addition of initiator is commenced atthe same point in time as the addition of the monomers and ended abouthalf an hour after the addition of the monomers. The initiator ispreferably added in a constant amount per unit time. Following the endof the addition of initiator, the reaction mixture is held atpolymerization temperature until (generally 1.5 hours) all of themonomers used have undergone essentially complete reaction. “Essentiallycomplete reaction” is intended to denote that preferably 100% by weightof the monomers used have been reacted but that it is also possible fora small residual monomer content of not more than up to about 0.5% byweight, based on the weight of the reaction mixture, to remainunreacted.

Preferably, the monomers for preparing the acrylic copolymers (A1) arepolymerized with not too high a polymerization solids, preferably with apolymerization solids of from 80 to 50% by weight, based on themonomers, and then the solvents are partially removed by distillation,so that the resulting acrylic copolymer solutions (A1) have a solidscontent of preferably from 100 to 60% by weight.

For use in the inventive coating material, the solids content of thesolutions of the acrylic copolymer solutions (A1) is adjusted with atleast one preferably aromatic-free solvent preferably to less than 60%by weight, particularly preferably less than 40% by weight, and inparticular less than 30% by weight.

Examples of suitable solvents are ethoxyethyl propionate and butylglycol.

The preparation of the acrylic copolymers (A1) for inventive use has nospecial features in terms of method but instead takes place with the aidof the methods which are customary and known in the field of polymersfor continuous or batchwise copolymerization under atmospheric orsuperatmospheric pressure in stirred vessels, autoclaves, tube reactorsor Taylor reactors.

Examples of suitable copolymerization processes are described in thepatents DE-A-197 09 465, DE-C-197 09 476, DE-A-28 48 906, DE-A-195 24182, EP-A-0 554 783, WO 95/27742 or WO 82/02387.

In accordance with the invention, Taylor reactors are advantageous.

Taylor reactors, which serve to convert substances under the conditionsof Taylor vortex flow, are known. They consist essentially of twocoaxial concentric cylinders of which the outer is fixed while the innerrotates. The reaction space is the volume formed by the gap between thecylinders. Increasing angular velocity ω_(i) of the inner cylinder isaccompanied by a series of different flow patterns which arecharacterized by a dimensionless parameter, known as the Taylor numberTa. As well as the angular velocity of the stirrer, the Taylor number isalso dependent on the kinematic viscosity ν of the fluid in the gap andon the geometric parameters, the external radius of the inner cylinderr_(i), the internal radius of the outer cylinder r_(o) and the gap widthd, the difference between the two radii, in accordance with thefollowing formula:

Ta=ω _(i) r _(i) dν⁻¹(d/r _(i))^(½)  (I)

where d=r_(o)−r_(i).

At low angular viscosity, the laminar Couette flow, a simple shear flow,develops. If the rotary speed of the inner cylinder is increasedfurther, then, above a critical level, alternately contrarotatingvortices (rotating in opposition) occur, with axes along the peripheraldirection. These vortices, called Taylor vortices, are rotationallysymmetric and have a diameter which is approximately the same size asthe gap width. Two adjacent vortices form a vortex pair or a vortexcell.

The basis of this behavior is the fact that, in the course of rotationof the inner cylinder with the outer cylinder at rest, the fluidparticles that are near to the inner cylinder are subject to a greatercentrifugal force than those at a greater distance from the innercylinder. This difference in the acting centrifugal forces displaces thefluid particles from the inner to the outer cylinder. The centrifugalforce acts counter to the viscosity force, since for the motion of thefluid particles it is necessary to overcome the friction. If there is anincrease in the rotary speed, there is also an increase in thecentrifugal force. The Taylor vortices are formed when the centrifugalforce exceeds the stabilizing viscosity force.

In the case of Taylor flow with a low axial flow, each vortex pairpasses through the gap, with only a low level of mass transfer betweenadjacent vortex pairs.

Mixing within such vortex pairs is very high, whereas axial mixingbeyond the pair boundaries is very low. A vortex pair may therefore beregarded as a stirred tank in which there is thorough mixing.Consequently, the flow system behaves as an ideal flow tube in that thevortex pairs pass through the gap with constant residence time, likeideal stirred tanks.

Of advantage in accordance with the invention here are Taylor reactorshaving an external reactor wall located within which there is aconcentrically or eccentrically disposed rotor, a reactor floor and areactor lid, which together define the annular reactor volume, at leastone means for metered addition of reactants, and a means for thedischarge of product, where the reactor wall and/or the rotor are or isgeometrically designed in such a way that the conditions for Taylorvortex flow are met over substantially the entire reactor length in thereactor volume, i.e. in such a way that the annular gap broadens in thedirection of flow traversal.

The proportion of the acrylic copolymer (A1) in the sol-gel clearcoatmaterial may vary very widely and is guided in particular by theintended flexibility of the inventive sol-gel coating producedtherefrom. There is an upper limit on the proportion; thus, it may notbe chosen so high that phase separation occurs in the coating materialof the invention, or the hardness and scratch resistance of the sol-gelcoating decrease too sharply. The skilled worker is therefore able todetermine the proportion which is optimal in each case, on the basis ofhis or her knowledge in the art, with or without the assistance ofsimple preliminary tests.

If desired, it is also possible to add an organic solvent such asaliphatic alcohol, such as methanol, ethanol, propanol, isopropanol orbutanol, an ether such a dimethoxyethane, an ester such as dimethylglycol acetate or methoxypropyl acetate, and/or 2-ethoxyethanol, or aketone such as acetone or methyl ethyl ketone, to the hydrolyzablesilane compounds (B).

If desired, organically modified metal alkoxides are also present. Theseare hydrolyzable metal alkoxides, it being possible to refer to thecorresponding and abovementioned groups R for the silanes in respect ofthe definition of the hydrolyzable groups. Preferred metal alkoxidesused are aluminum, titanium or zirconium alkoxides.

The sol-gel clearcoat material may comprise metal oxide nanoparticles.These nanoparticles are <50 nm. They may comprise, for example, Al₂O₃,ZrO₂ and/or TiO₂.

For the preparation of the sol-gel clearcoat material, the startingcomponents (B) are, for example, precondensed in the desired proportionwith a smaller amount of water than the amount requiredstoichiometrically for complete hydrolysis of all of the hydrolyzablegroups used. The substoichiometric amount of water is metered in suchaway to avoid local excess concentrations. This is done, for example, byintroducing the amount of water into the reaction mixture usingmoisture-laden adsorbents, e.g., silica gel or molecular sieves, hydrousorganic solvents, e.g., 80% ethanol, or salt hydrates, e.g., CaCl₂×6H₂O.Precondensation takes place preferably in the presence of a condensationcatalyst but in the absence of an organic solvent.

Suitable condensation catalysts include proton- or hydroxyl-ion-donatingcompounds and amines. Specific examples are organic or inorganic acids,such as hydrochloric acid, sulfuric acid, phosphoric acid, formic acidor acetic acid, and organic or inorganic bases such as ammonia, alkalimetal hydroxides or alkaline earth metal hydroxides, e.g., sodium,potassium or calcium hydroxide, and amines soluble in the reactionmedium, examples being lower alkyl amines or alkanol amines. Particularpreference is given in this context to volatile acids and bases,especially hydrochloric acid, ammonia and triethylamine, and also toacetic acid.

The precondensation is continued, for example, until the resultingprecondensate still has a liquid consistency. Preferably, it has asolids content of less than 80% by weight, with particular preferenceless than 60% by weight, and in particular less than 40% by weight.Since the precondensate coating material obtained is sensitive tohydrolysis, it should be stored, if necessary, under conditions whichexclude moisture.

The subsequent hydrolytic further condensation of the precondensate cantake place in the presence of at least the amount of waterstoichiometrically required to hydrolyze the remaining hydrolyzablegroups, but preferably with a superstoichiometric amount of water.Further condensation takes place preferably in the presence of one ofthe abovementioned condensation catalysts.

The precondensate coating material, or the coating material obtained byfurther condensation, which is also referred to as the stock coatingmaterial, may be used per se as sol-gel clearcoat material.

For this end use, however, it is preferably admixed with the acryliccopolymer solution (A1), provided the hydrolysis and precondensationhave not been conducted in the presence of the acrylic copolymer (A1).

Furthermore, for purposes of the preparation of the sol-gel clearcoatmaterial, it is preferred to add an additive solution, also referred toas primer, to the stock coating material.

This solution comprises at least one ethylenically unsaturated compoundcontaining at least one epoxide group. An example of a suitableethylenically unsaturated compound is glycidyl (meth)acrylate.

It further comprises at least one silane (B) containing at least onenonhydrolyzable group R which contains at least one epoxide group. Anexample of a suitable compound is 3-glycidyloxypropyltrimethoxysilane.

Not least, it comprises at least one adduct of at least one silane (B)containing at least one nonhydrolyzable group R, containing at least oneamino group, and at least one cyclic ethylenically unsaturateddicarboxylic anhydride. An example of a suitable silane (B) is3-aminopropyltriethoxysilane. Examples of suitable dicarboxylicanhydrides are maleic anhydride and itaconic anhydride.

The additive solution contains the ethylenically unsaturated compound,the silane (B) and the adduct in a weight ratio of (1to 10):(1 to 30):1,in particular (2 to 6):(10 to 20):l. The solids content of the additivesolution is preferably below 80% by weight, more preferably below 60% byweight, and in particular below 50% by weight.

The proportion of the additive solution in the sol-gel clearcoatmaterial may also vary widely. The skilled worker is able to determinethe proportion that is optimal in each case, on the basis of his or herknowledge in the art, with or without the assistance of simplepreliminary tests.

Particularly advantageous sol-gel clearcoat materials contain, based ineach case on their overall amount, from 5 to 20, preferably from 10 to15, and in particular from 10.5 to 14% by weight of the acryliccopolymer solution (A1), from 40 to 85, preferably from 45 to 80, and inparticular from 50 to 75% by weight of the stock coating material, andfrom 0.5 to 3, preferably from 1to 2, and in particular from 1.2 to 1.7%by weight of the additive solution.

It is of very particular advantage in this context, in accordance withthe invention, for the solids contents of the acrylic copolymer solution(A1), of the stock coating material and of the additive solution to bechosen such that in the sol-gel clearcoat material they are in a weightproportion of solids (A1): solids stock coating material: solidsadditive solution of

1to 10:30 to 60:1

preferably 2 to 8: 35 to 55:1, and

especially 2.5 to 6:40 to 50:1.

The sol-gel clearcoat material may further comprise at least one curingagent. Examples of suitable curing agents are quaternary ammoniumcompounds such as tetraalkylammonium salts, especiallytetramethyl-ammonium iodide. In the sol-gel clearcoat material, thecuring agent may preferably be present in an amount of from 0.001 to 1%by weight.

Furthermore, customary coatings additives may also be added to thesol-gel clearcoat material, examples being organic diluents, levelingagents, UV stabilizers, viscosity regulators, or antioxidants. It ispossible to use the same additives which are also used for customarycoating materials; by way of example, reference may be made to thecompounds mentioned in the powder slurry clearcoat material.

Chemical bonding is possible between the powder slurry clearcoatmaterial used and the sol-gel clearcoat material; in other words, boththe powder slurry clearcoat material and the sol-gel system contain ineach case functional groups which are able to react with one another(principle of “corresponding functional groups”). In the powder slurryclearcoat material, the corresponding functional group may be present,for example, in the crosslinking agent or, preferably, in the binder.

Examples of such suitable functional groups are hydroxyl, siloxane,anhydride, isocyanate, amine, epoxy, and carboxyl groups. The key factoris that a functional group is present firstly in the powder slurryclearcoat material and secondly in the sol-gel clearcoat material, achemical bond being possible between these two. Examples of functionalgroup pairs permitting such a chemical reaction are epoxide/carboxylgroups, epoxide/hydroxyl groups, functional groups containing reactive Hatoms/isocyanate group, such as amines/isocyanate groups andhydroxyl/isocyanate groups. Preference for use as a correspondingfunctional group is given to the pairing of epoxide group/carboxylgroup.

In accordance with the invention it is of advantage here if a certainfraction of the carboxyl-containing crosslinking agent b) or binder b)of the powder clearcoat reacts with a certain fraction of the epoxidegroups in the sol-gel clearcoat material.

The introduction of the corresponding functional group in the sol-gelclearcoat material may take place in the organic phase or in theinorganic phase. The corresponding functional group is to be chosen suchthat it is also present in the finished sol-gel clearcoat material.

For instance, the corresponding functional group may be introduced, forexample, by the nonhydrolyzable radical R of the silane (B). An exampleof a suitable silane (B) is 3-glycidyloxypropyltrimethoxysilane. Thefunctional group may, for example, also be introduced by way of theorganic phase, by adding corresponding monomers. For instance, anepoxide group in the case of monomers containing ethylenicallyunsaturated double bonds for the organic phase of the sol-gel clearcoatmaterial may be introduced, for example, by way of 2,3-epoxypropylmethacrylate.

The reaction of just a small portion of the corresponding functionalgroups with one another produces a network which, although in the formof a coarse mesh, is entirely adequate for the purpose of the invention.

Following the application of the powder slurry clearcoat material, it ispredried and induced to form a film. This takes place at a temperaturewhich, on the one hand, is above the film formation temperature of thepowder slurry clearcoat material and, on the other hand, is below thetemperature at which the crosslinking of this powder slurry clearcoatmaterial is complete. If T_(F) is the temperature at which the powderslurry clearcoat material begins to form a film, and T_(v) thetemperature at which the powder slurry clearcoat material has undergonecomplete crosslinking, then the temperature T at which, in accordancewith step A, the predrying and also the film formation take place isgoverned by:

T_(F)<T<T_(V)

The temperature T may be, for example, between 60° C. and 150° C.,preferably between 120° C. and 140° C., and in particular may besituated at 130° C., a period of from 5 to 30 min, preferably from 5 to15 min, and in particular 10 min, being an advantage in accordance withthe invention for the predrying and film formation. Before thispredrying and film formation at the temperature T, if desired, theapplied powder slurry clearcoat material is initially aerated at from 30to 60° C., preferably from 40 to 50° C., and in particular 50° C., forfrom 5 to 30 min, preferably from 5 to 15 min, and in particular 10 min.

In accordance with the invention it is important that the temperature Tis not so high that the crosslinking of the powder slurry clearcoatmaterial has come to an end; in other words, in step A) the powderslurry clearcoat material is cured only partly but not completely.

Subsequently, the sol-gel clearcoat material is applied to this powderslurry clearcoat material which has been predried and induced to form afilm. Application may take place by any technique known to the skilledworker. Preferably, sol-gel clearcoat material is applied by spraying.In this case the sol-gel clearcoat material is applied preferably as avery thin coating, e.g., <10 μm.

Subsequently, the powder slurry clearcoat film and the sol-gel clearcoatfilm are cured conjointly. This may be carried out, for example, attemperatures of above 85° C., preferably above 1300C and below 170° C.,more preferably below 160° C. The duration of baking may vary widely andmay be adapted outstandingly to the particular coating system present.In general, the duration of baking is between 10 min and 5 hours,preferably between 15 min and one hour.

If dodecane-1,12-dicarboxylic acid is used as crosslinking agent for thepowder slurry clearcoat material, then it is advantageous if thetemperature used in step A) corresponds approximately to the meltingpoint of dodecane-1,12-dicarboxylic acid. In that case the temperatureused in step A) differs preferably by not more than 5° C. from themelting point of dodecane-1,12-dicarboxylic acid.

The coated substrates produced by the process of the invention arenotable for very good scratch resistance coupled with very goodadhesion, even after exposure to condensation, between powder slurryclearcoat material and sol-gel clearcoat. The appearance too is good.The process of the invention is therefore particularly suitable for thecoating of vehicle bodies, especially automobile bodies, with multicoatsystems; for industrial coating, including the coil coating process; andfor furniture coating.

EXAMPLE AND COMPARATIVE EXPERIMENT C1 1. The Preparation of the StartingCompounds

1.1 The preparation of a glycidyl-containing acrylic resin as binder a)

21.1 parts of xylene were charged to an appropriate reaction vessel andheated to 130° C. The initiator, consisting of 4.5 parts of TBPEH(tert-butyl perethylhexanoate) and 4.86 parts of xylene, and the monomermixture, consisting of 10.78 parts of methyl methacrylate, 25.5 parts ofn-butyl methacrylate, 17.39 parts of styrene and 23.95 parts of glycidylmethacrylate, were metered into this initial charge at 130° C. over thecourse of four hours from two separate feed vessels. Subsequently, theresulting mixture was heated to 180° C., and the solvent was strippedoff in vacuo at less than 100 mbar. This gave the acrylic resin 1.1.

1.2 The preparation of a hydroxyl-containing acrylic resin as polyol c)

23.83 parts of xylene were charged to an appropriate reaction vessel andheated to 130° C. The initiator, consisting of 4.03 parts of TBPEH(tert-butyl perethylhexanoate) and 4.03 parts of xylene, and the monomermixture, consisting of 17.45 parts of methyl methacrylate, 14.09 partsof n-butyl methacrylate, 16.78 parts of styrene and 18.79 parts ofhydroxypropyl methacrylate, were metered into this initial charge at130° C. over the course of four hours from two separate feed vessels.Subsequently, the two feed vessels were rinsed out with 0.5 part ofxylene. Subsequently, the resulting mixture was heated to 180° C., andthe solvent was stripped off in vacuo at less than 100 mbar. This gavethe acrylic resin 1.2.

2. The Preparation of a Powder Clearcoat Material

62.8 parts of the acrylic resin 0.1, 13.5 parts of dodecanedicarboxylicacid, 5.0 parts of solvent-free tris(alkoxycarbonylamino)triazine, 14.8parts of acrylic resin 1.2, 2.0 parts of Tinuvin 1130 (UV absorber fromCiba-Geigy), 0.9 part of Tinuvin 144 (light stabilizer based on ahindered amine (HALS), from Ciba-Geigy), 0.4 part of Additol XL(leveling agent from Hoechst AG) and 0.4 part of benzoin (devolatilizer)were intimately mixed in a Henschel fluid mixer, extruded on a BUSS PLK46 extruder, and ground in a Hosokawa ACM 2 mill. The powder clearcoatmaterial was sieved off through a 125 micrometer sieve.

The solvent-free tris(alkoxycarbonylamino)triazine, for its part, wasobtained by removing the solvent from the commercial resin solution (51%strength in n-butanol, from Cytec) by distillation under reducedpressure at from 50 to 130° C. and discharging the resulting resin meltonto a pelletizing cooling belt or into a cooling pan.

This gave the powder clearcoat material 2.

3. The Preparation of a Powder Slurry Clearcoat Material

0.6 part of Troykyd D777 (defoamer from Troy Chemical Company), 0.6 partof Orotan 731K (dispersing aid from Rohm & Haas), 0.06 part of SurfynolTMN 6 (wetting agent from Air Products) and 16.5 parts of RM8 (nonionicassociative thickener based on polyurethanes, from Rohm & Haas) weredispersed in 400 parts of deionized water. Subsequently, 94 parts of thepowder clearcoat material 2. were introduced with stirring in smallportions. Subsequently, a further 0.6 part of Troykyd D777, 0.6 part ofOrotan 731K, 0.06 part of Surfynol TMN 6 and 16.5 parts of RM8 wereincorporated by dispersion. Subsequently, a further 94 parts of thepowder clearcoat material 2. were introduced with stirring, in smallportions.

The resultant mixture was ground in a sand mill for 3.5 hours. Thesubsequently measured average particle size was 4 μm. The powderclearcoat slurry was filtered through a 50 μm filter and admixed with0.05% by weight, based on its overall amount, of Byk 345 (leveling agentfrom Byk).

This gave the powder clearcoat slurry 3.

4. The Preparation of the Sol-gel Clearcoat Material for Inventive Use

4.1 The preparation of a stock coating material

An appropriate reaction vessel was charged with 30 parts of deionizedwater, 40 parts of ethyl glycol, 5 parts of 100% acetic acid, 66.5 partsof methyltriethoxysilane, and 3.5 parts of3-glycidyloxypropyltrimethoxysilane, and this initial charge was heatedwith stirring to 60° C. After a further 3 hours at 60° C., the reactionmixture was heated to 90° C. with stirring, and was held at thistemperature for 2 hours. Subsequently, 70 parts of the reaction mixturewere removed by azeotropic distillation. After cooling to roomtemperature, 5 parts of methoxypropyl acetate and 0.1 part of BYK 301(leveling agent from BYK) were added to the reaction mixture. This gavethe stock coating material 4.1.

4.2 The preparation of a polyacrylate for modifying the sol-gelclearcoat material

5 parts of acrylic acid, 95 parts of methyl methacrylate and 4 parts ofthe initiator TBPEH (tert-butyl perethylhexanoate) were polymerized in420 parts of ethyl glycol at 110° C. for two hours. This gave theorganically modifying solution 4.2.

4.3 The preparation of a primer (additive solution)

15 parts of ethyl glycol, 2 parts of 2,3-epoxypropyl methacrylate, 7.5parts of 3-glycidyloxypropyltrimethoxysilane, 0.5 part of an adduct ofmaleic anhydride and 3-aminopropyltriethoxysilane, and 0.1 part ofazodicarboxamide (GenitronR AZDN-M) were mixed with one another, withstirring, at 100° C. for six hours. This gave the primer 4.3.

The adduct, for its part, was prepared by reacting with one another 220parts of 3-aminopropyltriethoxysilane and 100 parts of maleic anhydride.

4.4 The preparation of the sol-gel clearcoat material

The sol-gel clearcoat material 4. was obtained by mixing with oneanother 14.3 parts of a stock coating material 4.1 (36% strength inethyl glycol), 2.14 parts of the solution 4.2, 2.7 parts ofmethoxypropyl acetate, 0.014 part of BYK 301, 0.3 part of the primer4.3, and 0.15 part of Tinuvin 329 (30% strength in toluene, lightstabilizer from Ciba-Geigy).

5. The Application of the Inventive Powder Clearcoat Slurry 3 (exampleand comparative example C1)

The powder clearcoat slurry 3. was applied using a so-called integratedsystem, which is described below for the metallic shade jungle green.

Using a gravity feed gun, a functional film of the coating materialEcoprime® from BASF Coatings AG was applied first of all to steel panelscoated cathodically with a commercially customary electrodepositioncoating material. After flashing off at room temperature for fiveminutes, a green metallic aqueous basecoat material (Ecostar® junglegreen from BASF Coatings AG) was applied in the same way to this filmand was subsequently predried at 80° C. for 10 minutes.

After the panels had been cooled, the powder clearcoat slurry 3. wasapplied in the same way, followed by predrying at 50° C. for 10 minutesand partial crosslinking at 130° C. for 10 minutes (example).

This process step was repeated with further panels, except that themulticoat system was baked at 150° C. for 30 minutes, resulting incomplete crosslinking (comparative experiment C1).

Subsequently, the sol-gel clearcoat material 4. was applied to thepanels of the example and of the comparative example Cl, and thenpredried at 50° C. for 10 minutes. Thereafter, the coating films werebaked at 150° C. for 30 minutes.

This resulted in two overall metallic coating systems in the junglegreen shade on the inventive and the noninventive test panels 5.

The thickness of the wet films was chosen so that, after baking, the drycoat thicknesses of the functional coat and of the metallic aqueousbasecoat were in each case 15 micrometers; the coat thickness of theclearcoat was 44 μm, and that of the sol-gel clearcoat was 8 μm.

6. The Testing of the Mechanotechnological Properties of the InventiveTest Panels 5. (example) and of the Noninventive Test panels 5.(comparative experiment C1)

6.1 Scratch resistance and adhesion of the sol-gel clearcoat material

The table gives an overview of the mechanotechnological tests and of theresults obtained in them.

TABLE Mechanotechnological properties of the inventive and of thenoninventive test panels 5. Test methods Scratch test to DBL 7399Example Comparative [Rating 0 to 5] 0 Experiment C1 0 Scratch test after240 hours of 0 0 constant condensation conditions (CCC) [Rating 0 to 5]Cross-hatch to DIN 53151 [2 mm] 0 5 [Rating 0 to 5] Cross-hatch after240 hours of 0 5 CCC and 24 hours of regeneration [Rating from 0 to 5]:0 = best; 5 = worst

On clearcoat materials which had been baked completely prior to theirovercoating with sol-gel coating (comparative experiment C1), therefore,there was complete delamination of the sol-gel clearcoat.

6.2 Scratch resistance by the brush test

The scratch resistance of the sol-gel clearcoat material on theinventive test panels 5. was assessed with the aid of the BASF brushtest described in FIG. 2 on page 28 of the article by P. Betz and A.Bartelt, Progress in Organic Coatings, 22 (1993), pages 27-37, albeitwith modification with regard to the weight used (2000 g instead of the280 g specified therein), assessment taking place as follows:

In the test, the film surface was damaged using a mesh fabric loadedwith a mass. The mesh fabric and the film surface were wetted copiouslywith a laundry detergent solution. The test panel was moved back andforward under the mesh fabric in reciprocal movements by means of amotor drive.

The test panels were prepared by applying first an electrocoat with afilm thickness of 18-22 μm, then a primer-surfacer with a film thicknessof 35-40 μm, then a black basecoat with a film thickness of 20-25 μm,and finally, by the procedure of the invention, the powder slurryclearcoat material with a film thickness of 40-45 μm and the sol-gelclearcoat material with a film thickness of 8 Am, each of which werecured. The panels, following application of the coating materials, werestored at room temperature for at least 2 weeks before testing wascarried out.

The test element was an eraser (4.5×2.0 cm, broad side perpendicular tothe direction of scratching) covered with nylon mesh fabric (No. 11, 31μm mesh size, Tg 50° C.). The applied weight was 2000 g.

Prior to each test, the mesh fabric was replaced, with the runningdirection of the fabric meshes parallel to the direction of scratching.Using a pipette, approximately 1ml of a freshly stirred 0.25% strengthPersil solution was applied in front of the eraser. The rotary speed ofthe motor was adjusted so that 80 double strokes were performed within aperiod of 80 s. After the test, the remaining washing liquid was rinsedoff with cold tap water and the test panel was blown dry usingcompressed air. The gloss to DIN 67530 was measured before and afterdamage (measurement direction perpendicular to the direction ofscratching).

It was found in this test that the gloss does not change at all as aresult of the loading, which is a convincing demonstration of theextremely high scratch resistance of the clearcoat of the invention.

What is claimed is:
 1. A process for producing a coated substrate,comprising applying a powder slurry clearcoat material to a substrate toform a coated substrate, subjecting the coated substrate to atemperature such that the powder slurry clearcoat material is predriedand induced to form a film which is only partly cured, said temperaturebeing above the film formation temperature of the powder slurryclearcoat material and below the temperature at which crosslinking ofthe powder slurry clearcoat material is complete, applying a sol-gelclearcoat material to the only partly cured powder slurry material film,and curing the sol-gel clearcoat material and the only partly curedpowder slurry material film conjointly such that a chemical bond resultsbetween them.
 2. The process of claim 1, wherein the powder slurryclearcoat material comprises epoxide groups and carboxyl groups, and thesol-gel clearcoat material comprises epoxide groups.
 3. The process ofclaim 2, wherein the powder slurry clearcoat material comprises at leastone binder containing epoxide groups, at least one carboxyl-containingcrosslinking agent, and an aqueous dispersion comprising at least onenonionic thickener and a dispersant in the form of a nonionicpolyurethane dispersion.
 4. The process of claim 3, wherein thecrosslinking agent is dodecane-1,12-dicarboxylic acid.
 5. The process ofclaim 4, wherein the temperature to which the coated substrate issubjected to corresponds approximately to the melting point ofdodecane-1,12-dicarboxylic acid.
 6. The process of claim 1, wherein thesol-gel clearcoat is applied in a film thickness of not more than 10 μm.7. A coated substrate prepared by the process of claim
 1. 8. The coatedsubstrate of claim 7, which is a vehicle body.
 9. A process for making avehicle, comprising using the coated substrate of claim
 7. 10. Theprocess of claim 1 wherein one or more coating compositions have beenapplied to the substrate prior to the application of the powder slurryclearcoat material.
 11. The process of claim 3, wherein the at least onebinder comprises a polyacrylate resin containing epoxide groups.
 12. Theprocess of claim 3 wherein the at least one carboxyl-containingcrosslinking agent is selected from the group consisting ofstraight-chain, aliphatic dicarboxylic acids, carboxy-functionalpolyesters, and mixtures thereof.